气体在水中的分子动力学模拟

Molecular Dynamics Simulation of Gases in Water

采用分子动力学（ＭＤ）模拟的方法在常温及工业应用背景条件下对ＣＨ４、ＮＨ３、ＣＯ２、Ｏ２这些气体在水中的结构及扩散情形进行了研究。ＭＤ模拟可以为这些涉及到气体在水中的工业应用情形的机理提供分子水平的解释，同时ＭＤ模拟还可为一些不易实验测定扩散性质的体系提供工程初步设计和过程开发所需的数据。

In many industrial processes, gas-water systems are frequently involved. CH4 is the prototype of nonpolar molecules, the investigation of its structural and dynamic properties in water is of great importance to further study the folding of proteins in water. NH_3 and CO_2 are related to the hydrolyze process of urea. To give a reasonable description of such a process, the diffusion coefficients of these gases in water are necessary. Supercritical water oxidation method is a newly-developed environmental technology to treat toxic organic waste water. To determine the desired diffusion properties of these systems experimentally is inconvenient. In this paper, the microstructure and diffusion properties of CH4, O2, CO2 and NH3 in water at room temperature and industrial operation conditions were studied by molecular dynamics simulation with SPC model of water. Good agreement of MD simulation results with experiments for diffusion coefficients of these systems at 298.15K was achieved. It is found that the methane molecule has a tendency to form a cage-like hydrate with the surrounding water molecules at 180K, while the diffusion coefficient of methane in low temperature water is one order magnitude lower than the one at 298.15K. The structural characters of CO2, and NH3 in water at 0.4MPa and 383.15 or 408.15K have little change compared to those at ambient condition. While the diffusion coefficients of such a process are several times larger than the one at room temperature. Larger diffusion coefficients are beneficial for accelerating the rates of urea hydrolyzing process. For the system of oxygen in supercritical water (703.15K and 30MPa), no obvious characteristic peak is found in the oxygen-water radial distribution functions, which indicate the solute is completely miscible with the solvent. The diffusion property of oxygen in supercritical water is several dozens higher than those at the ambient condition. Higher diffusion coefficients are favorable for speeding up the rates of supercritical water oxidation processes. The simulation results could provide valuable information to guide and optimize the practical operation and engineering design of relevant gas-water systems.